This invention relates to a method of directing a heated gas through a heat exchanger in a regenerative thermal oxidizer and into the combustion chamber to volatilize or combust any solids that may have accumulated within the heat exchange elements.
Regenerative thermal oxidizers ("RTO") are known, and are often utilized to remove volatile organic compounds ("VOC's") from an air stream. The air stream is typically process gas from another industrial process, such as a paint spray booth. The RTO removes the VOC's from the air stream by passing the process or "dirty" gas through a first previously heated heat exchanger and into a combustion chamber. The "dirty" gas is combusted and cleaned in the heat exchanger chamber. At the same time, a second heat exchanger is receiving heated clean air from the combustion chamber. After a period of time, the flow of cool dirty gas and heated clean gas are switched between the heat exchangers. Thus, the first heat exchanger that had previously been hot and was heating the dirty gas is switched to receiving the hot clean gas. The first heat exchanger is then again heated. The second heat exchanger that had been receiving the hot clean gas is switched to receiving the cool dirty gas and preheats that gas on its way to the combustion chamber. In this way, the regenerative thermal oxidizer continuously processes gas and efficiently removes impurities from a gas flow.
In many prior art RTO systems, a purge cycle is also included. When the heat exchangers are switched from receiving the dirty gas to the cool clean gas, there may sometimes be some residual dirty gas remaining in the heat exchanger. The clean gas is directed back to atmosphere, and no dirty gas should remain in the heat exchanger that begins to receive the clean gas. Thus, the prior art has included the purge cycle which drives residual dirty gas from a heat exchanger prior to that heat exchanger being switched to an outlet mode where it receives the clean gas. In many applications, RTO systems include a third heat exchanger such that the processing can continue at all times, with one heat exchanger being in an inlet mode receiving cool dirty gas, one heat exchanger being in an outlet mode receiving hot clean gas, and the third heat exchanger being in the purge mode.
Problems exist with such systems in that the heat exchange elements within the heat exchanger often accumulate organic solids from the air flow. The air flow containing the dirty gas tends to be in a first direction through the heat exchanger, and the air flow containing the clean gas is typically in an opposed direction. The heat exchange elements are often small particles of ceramic or other materials with good heat transfer properties. Such small particles or saddles, as they are typically known, provide numerous complex surface areas that can easily accumulate a good deal of organic waste. Thus, the heat exchanger elements will often accumulate organic solids. Further, the flow lines leading to the heat exchanger, and in particular the inlet manifold valves and flow lines often also build up accumulated solids.
The prior art has typically cleaned these heat exchangers by locking the valves in a first position such that the outlet gas continues to pass through a given heat exchanger for an unusual length of time. That outlet gas raises the temperature of the heat exchange elements to bake out any accumulated organic solids. However, this system has not been as efficient as desired. Moreover, since the cleaning gas is passing in a different direction than the direction at which the organic solids are placed on the heat exchange elements, it may not always be as effective as would be desirable.